1. Field of the Invention
The present invention relates to control of a vibration wave motor that causes an electromechanical energy conversion element to generate progressive vibrations in a vibrating member to thereby cause relative motion between the vibrating member and a moving member in contact with the vibrating member.
2. Description of the Related Art
A so-called vibration wave motor causes an electromechanical energy conversion element to generate vibrations in an elastic member to thereby actuate a moving member (rotary member) in contact with the elastic member. The vibration wave motor is used as an actuator which is capable of taking out a large actuating force (torque) at low speed.
For example, a so-called progressive wave-type vibration wave motor causes the elastic member to generate a progressive vibration wave by excitation, and continuously actuates the moving member in pressure contact with the elastic member. Such a vibration wave motor is capable of smoothly actuating the moving member (see e.g. Japanese Patent Laid-Open publication No. 2001-157473).
The vibrating member used in the vibration wave motor disclosed in Japanese Patent Laid-Open publication No. 2001-157473 is formed by an annular elastic member. A group of projections each having a comb-tooth shape is formed on one end face of the elastic member in an axial direction thereof, and a friction material is secured to a top surface of each projection of the group using an adhesive or the like. Further, an annular piezoelectric element as the electromechanical energy conversion element is secured to the other end face of the elastic member in the axial direction thereof using an adhesive or the like, and the piezoelectric element is formed with a pattern electrode.
The pattern electrode formed on the piezoelectric element is equally divided, according to the order of vibration modes to be excited in an annular portion of the vibrating member, into a number of electrodes which is four times as large as the order of vibration modes. AC voltages each having a substantially sinusoidal wave shape and displaced in time phase by 90 degrees from each other are sequentially applied to the electrodes.
When the AC voltage is applied to each electrode at a frequency near the natural frequency of the excited vibration mode, the elastic member is caused to resonate by a bending moment applied thereto by the expansion and contraction of the piezoelectric element. Vibrations (vibration modes) excited by the AC voltages displaced in time phase by 90 degrees from each other are identical in wave shape but different in phase, so that a progressive vibration wave (progressive wave) is generated by synthesizing the vibrations.
There have been proposed various drive circuits for driving the above-described progressive wave-type vibration wave motor (see e.g. Japanese Patent Laid-Open publication No. 2002-176788). FIG. 7 is a schematic diagram of a drive circuit for driving the vibration wave motor, which is disclosed in Japanese Patent Laid-Open publication No. 2002-176788. In this drive circuit, a switching circuit formed by MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) 22 to 29 is on/off controlled by pulses generated by a pulse generating circuit, not shown, whereby an AC voltage is generated in center-tapped transformers 30 and 31. This sequentially applies the AC voltages displaced in time phase by 90 degrees from each other to terminals 32 to 35 connected to the secondary side which correspond to A(+) phase, B(+) phase, A(−) phase and B(−) phase, respectively.
In general, the speed control of the vibration wave motor by the drive circuit configured as above is performed by controlling the frequency of the AC voltage as an input signal. FIG. 8 is a characteristic diagram showing the relationship between the frequency of the AC voltage and the rotational speed of the vibration wave motor. In FIG. 8, the horizontal axis represents the frequency of the AC voltage, which becomes higher toward the right as viewed in the figure, and the vertical axis represents the rotational speed (actuation speed) of the vibration wave motor, which becomes higher upward as viewed in the figure.
In general, the vibration wave motor is driven as follows: First, the frequency of the AC voltage is set to a value sufficiently higher than a resonance frequency f, of the vibration wave motor and then the vibration wave motor is started. Then, the frequency of the AC voltage is progressively made closer to the resonance frequency fr (the frequency is made lower), to thereby accelerate the vibration wave motor, whereafter the frequency is progressively made away from the resonance frequency (the frequency is made higher), to thereby decelerate the vibration wave motor. Here, in accelerating the vibration wave motor, if the frequency of the AC voltage becomes lower than the resonance frequency fr, the rotational speed of the vibration wave motor suddenly drops. Therefore, the frequency of the AC voltage is set to a range within which it is not lower than the resonance frequency fr.
However, as can be understood from FIG. 8, the frequency of the AC voltage applied to the vibration wave motor and the rotational speed of the vibration wave motor are in a nonlinear relationship. Further, frequency-rotational speed characteristics shown in FIG. 8 vary not only depending on differences between individual vibration wave motors of the same model but also by a variation in load applied to the vibration wave motor and a change in the temperature of the vibration wave motor. For this reason, even when the difference between an actual rotational speed of the vibration wave motor at a certain time point and a target rotational speed thereof at the same time point (hereinafter referred to as “the rotational speed difference”) is calculated, it is difficult to unconditionally determine a frequency to be set, based on the rotational speed difference.